The investigations proposed here seek to gain understanding of the physiology of host defense against bacterial and fungal infections, and of the biochemical basis for the predisposition to bacterial infection in certain disease states. Mechanisms involved in the interaction of bacteria and polymorphonuclear and mononuclear phagocytes and complement activated through the alternative pathway will be studied in the following specific projects: (1) Studies of the conversion of superoxide anion and hydrogen peroxide to hydroxyl radical in phagocytic cells. (2) Purification of the enzyme or enzymes responsible for the conversion of oxygen to superoxide anion and other metabolites in neutrophils. (3) Studies of the molecular basis of the in vitro activation of cultured macrophages by bacterial products and adjuvants. (4) Studies of the enzymic basis for the increased oxidative metabolic activity manifested by elicited and activated macrophages. (5) Studies of the interaction between candida species and mononuclear phagocytes as related to the pathogenetic capacity of the different fungal species.